incr.comp.: Let the compiler ignore incompatible incr. comp. cache artifacts

[michaelwoerister]

Implements #35720.

cc @nikomatsakis

[rust-highfive]

r? @eddyb

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[eddyb]

@bors r+

[bors]

📌 Commit 1e5c253 has been approved by eddyb

[sophiajt]

I'm seeing travis fail with this:

error: environment variable `CFG_VERSION` not defined
  --> src/librustc_incremental/persist/file_format.rs:38:37
   |
38 | const RUSTC_VERSION: &'static str = env!("CFG_VERSION");
   |                                     ^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

Build failed, waiting for other jobs to finish...
error: Could not compile `rustc_incremental`.

[eddyb]

Blargh, I should've seen that coming. @bors r-

[eddyb]

I thought you had a reason for not following suit, but it appears rustc_incremental is also tested.

[michaelwoerister]

Ah yes, I ran across this and then forgot about again. Should be fixed now (but let's wait for travis).

[eddyb]

@bors r+

[bors]

📌 Commit 263ba92 has been approved by eddyb

[bors]

⌛ Testing commit 263ba92 with merge 8467e8d...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-cargotest, auto-linux-64-cross-freebsd, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-opt-rustbuild, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-opt, auto-mac-64-opt-rustbuild, auto-mac-cross-ios-opt, auto-win-gnu-32-opt, auto-win-gnu-32-opt-rustbuild, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-cargotest, auto-win-msvc-64-opt, auto-win-msvc-64-opt-rustbuild
Approved by: eddyb
Pushing 8467e8d to master...